Large petroleum and chemical companies, governments, including the US Department of Homeland Security, foreign governments and other corporations require safety systems to prevent catastrophic events caused by human failure, natural consequences of deteriorating conditions, acts of God, sabotage and the like. Unfortunately, recent failures in current safety solutions have failed to prevent catastrophic events, costing human lives, environmental disasters and billions of dollars in lost productivity. Security and safety hazards to critical infrastructure increasingly cost hundreds of millions to billions of dollars in losses. Historically, separate costly and complicated systems address security threats and safety hazards. The Department of Homeland Security (DHS) Maritime Transportation Security Act (MTSA) and the Chemical Facility Anti-Terrorism Standards (CFATS) currently mandate securing the nation's petrochemical infrastructure against security threats. The disastrous refinery explosion in Texas in 2005 caused by sensor malfunctions highlights the lack of appropriate security and safety systems for monitoring refineries and chemical plants for safety hazards and conditions. Both onshore and offshore critical infrastructure assets are covered by these pieces of legislation.
The term Safety Integrity Level (SIL) is defined as a relative level of risk-reduction provided by a safety function, or to specify a target level of risk reduction. Four SIL levels are defined, with SIL4 being the most dependable and SIL1 being the least. A SIL is determined based on a number of quantitative factors in combination with qualitative factors such as development process and safety life cycle management. One problem with SIL is that the requirements for a given SIL are not consistent among all of the functional safety standards.
The SIL requirements for hardware safety integrity are based on a probabilistic analysis of a situation. Generally, devices in a system should have less than the specified probability of dangerous failure and have greater than the specified safe failure fraction. Generally the statistics are calculated by performing a Failure Modes and Effects Analysis (FMEA). The actual targets required vary depending on the likelihood of a demand, the complexity of the device(s), and types of redundancy used.
PFD (Probability of Failure on Demand) and RRF (Risk Reduction Factor) for different SILs as defined in IEC61508 are exemplary:
TABLE 1SILPFDRRF10.1-0.0110-10020.01-0.001100-100030.001-0.0001 1000-10,00040.0001-0.0000110,000-100,000
The SIL requirements for systematic safety integrity define a set of techniques and measures required to prevent systematic failures from being designed into the device or system such as in a refinery or plant or base. These requirements can either be met by establishing a rigorous development process, or by establishing that the device has sufficient operating history to argue that it has been proven in use.
Electric and electronic devices can be certified for use in functional safety applications according to IEC 61508, providing application developers the evidence required to demonstrate that the application including the device is also compliant.
IEC 61511 is an application specific adaptation of IEC 61508 for the Process Industry sector and is used in the petrochemical and hazardous chemical industries, and others.
A problem with the different standards and SIL requirements and an unmet need in the industry is a cost efficient fault detection system appropriate for diverse applications.